Arrow functions' lexical binding is not working correctly

[smfoote]

The arrow function is supposed to bind to the scope in which it is defined (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions), and the this of an arrow function should not be able to change. As shown in the code below (run with the --harmony_arrow_functions), the arrow functions are not correctly binding.

var obj = {
  init: function() {
    this.handle(this.cb);
  },
  cb: val => {
    console.log(this.val);
    console.log(val);
    return val + this.val;
  },
  handle: function(cb) {
    console.log(cb(3));
  },
  val: 5
};

console.log('calling cb directly');
console.log(obj.cb(3));
// 5
// 3
// 8

console.log('calling cb via init');
obj.init();
// undefined
// 3
// NaN

[phpnode]

@smfoote this is a V8 bug, the arrow function implementation in the current V8 is not spec compliant and shouldn't be used. If you want/need such features, you'll have to use something like 6to5 for now.

[bnoordhuis]

@smfoote A general observation: if you have to pass a flag to enable a language feature in V8, the feature isn't stable yet.

[smfoote]

Thanks, all. I was simply trying to help by pointing out that the feature is not yet up to spec. I won't submit any other issues for features hidden by flags.

[bnoordhuis]

@smfoote That's appreciated. :-) Most things V8 are not under our direct control, though.